1. Field of the Invention
The present invention relates to a winding-type or laminated-type metallized plastic film and a film capacitor. More particularly, the present invention relates to a metallized plastic film and a film capacitor, improved in electrical property and safety of a film capacitor by restricting self-heating of the film capacitor and decreasing capacitance reduction rate caused by the operation of a fuse part and also improved in compactness and thermal resistance for use even at a high temperature.
2. Description of the Related Art
In general, electric capacitors, low-voltage power capacitors, electronic capacitors and so on are used in various fields of industry. Such capacitors usually adopt, as a dielectric, a plastic film which is made of polyethylene terephthalate resin, polypropylene resin, polyethylene naphthalate resin, polycarbonate resin or the like. To provide electrodes in those capacitors, a metallized plastic film formed by vapor-depositing a vapor-deposited metal such as zinc, aluminum, and an aluminum alloy on one or both sides of a plastic film under a high vacuum and another metallized plastic film in which margins are facing the opposite directions are wound as one group.
In the above wound element, zinc or a zinc alloy is sprayed to both sides to withdraw the electrodes, and electrode leading wires are connected by spot-welding or soldering and then insulated by an insulating material as housed in an outer case.
However, the above film capacitor causes a large volume and a high cost since a dedicated safety device is built in the outer case to guarantee safety in use and be used even under a high voltage. Also, when an inner voltage occurs, since the safety device is operated, the performance of the capacitor is deteriorated.
In order to overcome the aforementioned problems, a method has been introduced, which suggests use of a safety metallized plastic film or a pattern-metallized plastic film in the capacitor. According to the method, the vapor-deposited metal is split by a splitting part which is formed by applying a release agent such as oil to a surface of the plastic film by a predetermined form, before the vapor-deposition of the metallized plastic film, such that the metal is not vapor-deposited on the part applied with the release agent. Each split electrode is formed with a fuse part having a narrow width and connected to the vapor-deposited metal.
A general conventional metallized plastic film 8 shown in FIG. 1 is formed by vapor-depositing an electrode metal 2 to a plastic film 1. A split electrode of the electrode metal 2 is not provided. Here, since a temperature fuse or a current fuse for improving safety of the capacitor, or a dedicated safety device which interrupts power supply to the film capacitor by gas generated from an accident of the film capacitor is further employed, or since a thicker metallized plastic film which is 3˜5 μm thicker than the normal, the total volume and the price of the film capacitor are increased.
In another conventional metallized plastic film 8 as shown in FIG. 2a, the vapor-deposited metal 2 is split by a T-shape splitting part 3. More particularly, the electrode metal is vapor-deposited on the plastic film 1, and a split electrode 5 is formed by the T-shape splitting part 3, thereby forming one fuse part 4 at the vapor-deposited metal 2.
When insulation breakage occurs within an area of the split electrode 5, the fuse part 4 cuts off the film capacitor from the power supply by dispersing the vapor-deposited metal thereof using a current caused by the insulation breakage. Therefore, although capacitance of the film capacitor is reduced as much as the area of the split electrode 5, explosion of the film capacitor can be prevented, thereby enabling continuous performance of the film capacitor.
Additionally, the metallized plastic film 8 further includes a margin part 6 free of the vapor-deposited metal 2, being continuously formed in a length direction thereof, and a sprayed metal contacting part 7 formed at the opposite end with respect to a width direction.
According to the above structured metallized plastic film 8, since the fuse part 4 is in the vicinity of the sprayed metal contacting part 7, heat generated from both the fuse part 4 and the sprayed metal contacting part 7 are focused, thereby increasing the temperature of the film capacitor while deteriorating insulation efficiency at the heat-focused part. As a result, performance of the fuse part 4 may not be stably maintained.
Furthermore, if the width of the metallized plastic film 8 is greater than 40 mm, the area of the split electrode 5 is increased. Accordingly, the width of the fuse part 4 is increased, which may cause malfunction of the fuse part 4. Consequently, the film capacitor cannot operate as desired.
However, when the fuse part 4 is well operated on the other hand, capacitance of the film capacitor is greatly reduced. To this end, still another metallized plastic film 8 having two or three fuse parts 4 as shown in FIG. 2b has been suggested. However, this structure is also ineffective in guaranteeing the stable operation. Also, such great reduction of capacitance of the film capacitor during the operation of the fuse part 4 still occurs due to a large area of the split electrode 5.
In still another conventional metallized plastic film 8 as shown in FIG. 3, the vapor-deposited metal 2 is split into a plurality of diamond shapes. Respective ends of the split electrodes 5 formed by the splitting part 3 are interconnected through the fuse part 4.
According to this structure, however, electricity applied to the film capacitor flows toward the margin part 6 passing through the sprayed metal contacting part 7 of the metallized plastic film 8. Therefore, a plurality of the split electrodes 5 are to be arranged along the width of the metallized plastic film 8, thereby increasing the number of the fuse part 4, and this causes a bottleneck situation at the fuse parts 4, thereby focusing heat to the fuse parts 4. To this end, the structure shown in FIG. 3 is inappropriate for use under a high voltage.
Still another conventional metallized plastic film 8 is shown in FIG. 4a. Referring to FIG. 4a, the split electrodes 5 are formed to occupy only about a half width of the vapor-deposited metal 2 and split by the T-shape splitting parts 3 each comprising four fuse parts 4. Compared to in the conventional metallized plastic film 8 shown in FIG. 2, the capacitance reduction caused by the operation of the fuse parts 4 is relatively smaller in this structure of FIG. 4a. However, since a pitch interval between the respective split electrodes 5 is great, the whole area of the metallized plastic film 8 is increased. As a result, the capacitance of the film capacitor also greatly reduces when the fuse parts 4 operate.
In addition, when the position of the fuse parts 4 is focused to the middle part of the metallized plastic film 8 and two sheets of the metallized plastic film are wound as one group as shown in FIG. 4b, the fuse parts 4 of the metallized plastic film 8 are focusedly arranged in the middle part, thereby causing focus of heat at the fuse part due to a bottleneck situation of the electric current. Accordingly, a withstanding voltage of the film capacitor is deteriorated, thereby restricting use of the film capacitor.
As shown in FIG. 5, still another conventional metallized plastic film 8 includes the split electrodes 5 having a diamond shape and occupying about a half of the vapor-deposited metal 2 with respect to the width direction. In comparison with the conventional structure shown in FIG. 3, this metallized plastic film 8 is capable of restraining the temperature increase by self-heating of the film capacitor by reducing the number of the fuse parts 4 in the width direction. However, a fuse part 4a adjoining the vapor-deposited metal 2 free of the split electrodes 5 cannot normally function as a fuse since having a great width. Therefore, safety of the film capacitor is not guaranteed.